Green fluoropolymer tube and endovascular prosthesis formed using same

ABSTRACT

The present invention is directed to a green fluoropolymer tube which has a wall thickness of about 250 microns or less, preferably about 200 microns or less, and more preferably about 125 microns or less. With such thinness, the green tube may be formed into a tube of expanded fluoropolymer having a wall thickness of about 250 microns or less, preferably about 200 μm or less and more preferably about 125 microns or less. Such an expanded fluoropolymer tube is particularly well-suited for use as an endovascular prosthesis, such as a graft, or in a stent-graft, because of its flexibility and strength.

FIELD OF THE INVENTION

[0001] The present invention relates generally to a vascular prosthesisthat is used to repair and/or replace or otherwise treat a vasculardefect. More particularly, the present invention relates to anexpandable prosthesis that incorporates extruded tubular members havingminimal wall thickness. Such thin-walled tubular members permitconstruction of graft and stent-graft devices having very small outerdiameters that are able to navigate tortuous vasculature withoutcompromising structural integrity.

BACKGROUND OF RELATED TECHNOLOGY

[0002] Techniques have been developed in the prior art to formthin-walled polytetrafluoroethylene (PTFE) tubes. For example, U.S. Pat.No. 4,925,710 to Buck et al., discloses a method of coating a core withPTFE, wherein, after sintering, the PTFE coating is removable from thecore to form a tube. Automotive cables and medical catheters aredisclosed as useful products. No disclosure is provided for uniformityof wall thickness nor the production of the tubes in the absence of thecore. Tubes having wall thicknesses of 0.002 inches (0.051 mm) and less,which are formed on solid cores, are disclosed.

[0003] Other techniques have been developed in the prior art whereporous expanded PTFE (ePTFE) is prepared in tape or film form, androlled or wrapped into tubular form with overlapping edges being bondedand/or sintered. See, e.g., U.S. Pat. No. 4,791,966 to Eilentropp(discloses wrapping a PTFE tape about a mandrel); U.S. Pat. No.6,027,779 to Campbell et al. (discloses overlaying two pieces of ePTFEfilm in forming a tube). Although these methods have producedthin-walled tubes having wall thicknesses of 300 μm and less, thesethin-walled tubes are formed of ePTFE and are not susceptible to furtherexpansion (and thus further wall reduction). In addition, these tubesinclude one or more seams.

[0004] With typical prior art PTFE tube extrusion, a PTFE billet ispre-formed and loaded into a ram extruder, resulting in a relativelythick-walled tube extrudate (typically having a wall thickness in therange of 0.010-0.017 inches (standard green tube for vascular graft)).Although the extrudate may be slightly tensioned during take-up afterextrusion, the extruded tube is “green”. (As used herein, a “green”fluoropolymer tube, such as a PTFE tube, is unsintered and eitherunexpanded or minimally expanded due to the manufacturing process suchas, e.g., being subjected to tension during take-up after extrusion.) Inaddition, extruded green PTFE typically contains a lubricant, which isvolatilized. Thereafter, the tube may be subjected to further processes,including heating, sintering and expansion, with expansion resulting inreducing the thickness of the tube wall to a range of about 0.010 toabout 0.017 inches, with a range of about 100% to about 500% elongation.As a result, a seamless, thin-walled tube of ePTFE may be formed.

[0005] Conventional PTFE extrusion methods have had difficulty inproducing thin-walled tubes for use as medical implants, such asvascular prostheses. For example, U.S. Pat. Nos. 5,700,285; 5,735,892and 5,810,870 to Myers describe these difficulties, including thedifficulty of maintaining the uniformity of wall thickness ofthin-walled extruded PTFE tubes. Consequently, these patents aredirected to producing thin-walled vascular tubes using thin PTFE sheetswhich are wrapped to form a tube.

SUMMARY OF THE INVENTION

[0006] To overcome deficiencies of the prior art, a green fluoropolymertube is provided having a wall with a generally uniform thickness in therange of about 250 μm or less, preferably about 200 μm or less, and,more preferably, about 125 μm or less. Advantageously, from such athin-walled green tube, a tube of expanded fluoropolymer can be formedhaving a wall thickness in the range of about 250 μm or less, preferablyabout 200 μm or less, and more preferably about 125 μm or less.

[0007] In a preferred embodiment, the green fluoropolymer tube isextruded, and therefore, is seamless. Accordingly, a seamless expandedfluoropolymer tube, i.e. ePTFE is also formed from the green tube and isparticularly well-suited for forming a prosthetic member, particularly,an endovascular prosthetic member, such as a graft. In addition, theprosthetic member can include multiple concentric layers of such tubesand can be used in combination with stents to form a stent-graft, i.e.,wherein a distensible member, such as a balloon-expandable or aself-expanding stent, is attached to or otherwise integrated with thegraft for structural support. It is preferred that the fluoropolymer bePTFE due to its inert, biologically-compatible, and porouscharacteristics.

[0008] Because of the thinness of the extruded green fluoropolymer tube,precise extrusion equipment is required, such as that disclosed incopending U.S. Application Ser. No. (Attorney Docket: 760-29), which isalso assigned to the assignee herein. Although other apparatuses may beused, it must be noted that due to the thinness of the green tube, itdoes not have inherent structural strength and integrity to supportitself in a hollow state, i.e., to maintain its lumen open and avoidkinking and collapsing, in whole or in part, when the green tube is“wet” immediately after being extruded. To provide support, apressurized stable medium, preferably air, is injected into the lumenduring extrusion to provide support.

[0009] In one particularly desirable aspect of the invention there isincluded a stent-graft device which includes concentric tubular layersof extruded thin-wall green PTFE which has been expanded and whichincorporate a stent between the concentric layers. The concentrictubular layers are desirably laminated together by heat, adhesive orother means, desirably through the openings in the stent. Alternatively,each layer may be adhered independently to the stent.

[0010] These and other features of the invention will be betterunderstood through a study of the following detailed description andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is an end view of a prosthesis formed in accordance withthe subject invention;

[0012]FIG. 2 is an exploded view of the prosthesis;

[0013]FIG. 3 is an end view of a green tube formed in accordance withthe subject invention; .

[0014]FIG. 4 and 5 are top and side views on an extrusion apparatuswhich may be used for forming the green tube;

[0015]FIG. 6 is a perspective view of an extrusion mold assembly usedwith the extrusion apparatus;

[0016]FIG. 7 is an enlarged view of a die and mandrel used with theextrusion apparatus;

[0017]FIG. 8 is a side view of a truss member used with the extrusionapparatus; and,

[0018]FIG. 9 is a flow chart setting forth a process for producing aprosthesis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] One aspect of the present invention is directed to a greenfluoropolymer tube which has a wall thickness of about 250 μm or less,preferably about 200 μm or less, and more preferably about 125 μm orless. With such thinness, the green tube may be formed into a tube ofexpanded fluoropolymer having a wall thickness of about 250 μm or less,preferably about 200 μm or less, and more preferably about 125 μm orless. Such an expanded fluoropolymer tube is particularly well-suitedfor use as an endovascular prosthesis, such as a graft, or in astent-graft, because of its flexibility and strength.

[0020] Now referring to the figures, wherein like elements areidentically numbered, the configuration and operation of the presentinvention can be described.

[0021] The present invention is desirably used in vascular prostheseshaving extremely thin walls, an example of which is shown in FIGS. 1 and2. As is evident from FIG. 1, prosthesis 50 is a tubular vascularconduit having an extruded luminal sheath 52 and, optionally, alsoincludes an extruded exterior sheath 54. Each sheath 52, 54 desirablyhas a thickness of less than or equal to about 250 μm, and morepreferably less than or equal to about 200 μm, and more preferably lessthan or equal to about 125 μm.

[0022] In accordance with the subject invention, the sheaths 52, 54 areformed of expanded fluoropolymer, preferably ePTFE. In addition, thesheaths 52, 54, either one or both, are expanded from a greenfluoropolymer tube 1, shown in FIG. 3. Preferably, the green tube 1 isformed of PTFE. In addition, as described in more detail below, thegreen tube 1 is formed by extrusion and, thus, includes a wall 2, whichis seamless. The resulting sheaths 52, 54 are also, advantageously,seamless. Even though techniques had been developed in the prior art toseal seams in wrapped or rolled ePTFE tubes, the existence of such seamsis not necessarily desirable and requires additional manufacturing stepsas compared to tube extrusion techniques.

[0023] The wall 2 of the green tube 1 has a thickness T in the range of250 μm and less. The thickness T is generally uniform throughout thewall 2 of a single extrudate. It is preferred that the thickness T berelatively uniform at generally all points in a single of the green tube1. For example, with a green tube 1 having a wall thickness T in therange of 80-120 μm, it is desirable to limit variations in the wallthickness T to 5 μm or less. Other tolerances in the thickness T arepossible. Preferably, the green tube 1 is cylindrical with an innersurface 4 of the wall 2 being formed about a diameter D₁ in the range ofabout 1-14 mm, and up to 35 mm, and an outer surface 6 being formedabout a diameter D₂ in the range of about 1- 14 mm, and up to 35 mm.Hollow lumen 8, with a relatively constant cross-section, is defined bythe wall 2.

[0024] Prosthesis 50 may optionally further include a distensible member56, preferably disposed between sheaths 52 and 54. As further shown inFIG. 2, distensible member 56 includes interstices 58 through whichsheaths 52 and 54 adhere to one another at bonded regions 60. At leastone of luminal sheath 52 and exterior sheath 54 may be flush with anextent of distensible member 56 or centered relative thereto to revealat least one open portion of the distensible member thereby. Althoughboth of sheaths 52 and 54 appear as extruded tubes that slide overdistensible member 56 during assembly of prosthesis 50, either sheath52, 54 may be a wrapped or rolled film or sheet having its opposingedges overlapped and secured to one another to form a tubular structure.

[0025] Assembly of prosthesis 50 is effected by first extruding adesired polymeric composition in a tubular profile, cutting such profileto a desired length, and working the cut profiles, (e.g., expanding;sintering) to obtain either or both of sheaths 52 and 54. Distensiblemember 56 is insertably retained within exterior sheath 54 such that anouter circumferential surface 56 a of distensible member 56 contiguouslycontacts an interior circumferential surface 54 a of sheath 54. Luminalsheath 52 is similarly insertably retained in distensible member 56 suchthat an outer surface 52 a of sheath 52 contiguously contacts aninterior circumferential surface 56 b of distensible member 56. It ispreferable that neither of sheath 52 or 54 be bonded to distensiblemember 56, thereby providing distensible member 56 with requisitefreedom of movement without adversely affecting the integrity of sheaths52 and 54.

[0026] Either or both of the luminal and exterior sheaths 52, 54 may beprovided with an adhesive thereon that permits adherence of thepolymeric sheaths to one another through interstices 58, therebyavoiding adherence of distensible member 56 to either or both of thesheaths 52, 54. The adhesive may be a thermoplastic adhesive and morepreferably, a thermoplastic fluoropolymer adhesive such as FEP. Asuitable adhesive provides a substantially integral and sealedprosthesis 50 without significantly reducing longitudinal and/or axialcompliance. Alternatively, the two sheaths 52, 54 may be affixed byplacing them under pressure and heating them above the softening ormelting point of the constituent polymeric composition from which thetubes are formed to cause them to thermally adhere to one another.

[0027] As is known in the art, one or both of sheaths 52 and 54 may beimpregnated with one or more therapeutic and pharmacological substancesprior to implantation of the prosthesis 50 for controlled release overan extended duration. It is anticipated that prosthesis 50 can bepartially or wholly coated with hydrophilic or drug delivery-typecoatings which facilitate long-term healing of diseased vessels. Such acoating is preferably bioabsorbable, and is preferably a therapeuticagent or drug, including, but not limited to, anti-thrombogenic agents(such as heparin, heparin derivatives, urokinase, and PPack(dextrophenylalanine proline arginine chloromethylketone));anti-proliferative agents (such as enoxaprin, angiopeptin, or monoclonalantibodies capable of blocking smooth muscle cell proliferation,hirudin, and acetylsalicylic acid); anti-inflammatory agents (such asdexamethasone, prednisolone, corticosterone, budesonide, estrogen,sulfasalazine, and mesalamine);antineoplastic/antiproliferative/anti-miotic agents (such as paclitaxel,5-fluorouracil, cisplatin, vinblastine, vincristine, epothilones,endostatin, angiostatin and thymidine kinase inhibitors); anestheticagents (such as lidocaine, bupivacaine, and ropivacaine);anti-coagulants (such as D—Phe—Pro—Arg chloromethyl keton, an RGDpeptide-containing compound, heparin, antithrombin compounds, plateletreceptor antagonists, anti-thrombin anticodies, anti-platelet receptorantibodies, aspirin, prostaglandin inhibitors, platelet inhibitors andtick antiplatelet peptides); vascular cell growth promotors (such asgrowth factor inhibitors, growth factor receptor antagonists,transcriptional activators, and translational promotors); vascular cellgrowth inhibitors (such as growth factor inhibitors, growth factorreceptor antagonists, transcriptional repressors, translationalrepressors, replication inhibitors, inhibitory antibodies, antibodiesdirected against growth factors, bifunctional molecules consisting of agrowth factor and a cytotoxin, bifunctional molecules consisting of anantibody and a cytotoxin); cholesterol-lowering agents; vasodilatingagents; and agents which interfere with endogenous vascoactivemechanisms. While the foregoing therapeutic agents have been used toprevent or treat various conditions, they are provided by way of exampleand are not meant to be limiting, as other therapeutic drugs may bedeveloped which are equally applicable for use with the presentinvention.

[0028] Although a wide variety of distensible members may be used, FIG.2 shows one particular distensible member 56, a stent, which may beemployed in prosthesis 50. The particular stent shown in FIG. 2 is morefully described in commonly assigned U.S. Pat. No. 5,693,085 to Buirgeet al. and the disclosure of U.S. Pat. No. 5,693,085 is incorporated byreference herein. The stent may be an intraluminally implantable stentformed of a metal such as stainless steel or tantalum, atemperature-sensitive material such as Nitinol or alternatively formedof a superelastic alloy or suitable polymer. Multiple struts 62interconnect adjacent columns 64 to thereby define a plurality of theinterstices 58 throughout the tubular configuration and further define acentral open passage 66 therethrough. Struts 62 and columns 64 providethe distensible member 56 with a generally elongate tubularconfiguration which is radially expandable after implantation in a bodyvessel. The distensible member 56 may be formed by etching or cutting apattern from a tube or, alternatively, formed by etching a pattern intoa material or mold and depositing stent material in the pattern, such asby chemical vapor deposition or the like.

[0029] Although a particular stent construction is shown and describedwith reference to the present invention, various stent types and stentconstructions may be employed in the present invention as thedistensible member 56 for the use anticipated herein. Among the varioususeful distensible members 56 include, without limitation,self-expanding stents and balloon expandable stents. The stents may becapable of radially contracting as well. Self-expanding stents includethose that have a spring-like action which causes the stent to radiallyexpand or stents which expand due to the memory properties of the stentmaterial for a particular configuration at a certain temperature. Othermaterials are of course contemplated, such as stainless steel, platinum,gold, titanium, tantalum, niobium, and other biocompatible materials, aswell as polymeric stents.

[0030] The configuration of the distensible member 56 may also be chosenfrom a host of geometries. For example, wire stents can be fastened in acontinuous helical pattern, with or without wave-like forms or zigzagsin the wire, to form a radially deformable stent. Individual rings orcircular members can be linked together such as by struts, sutures, orinterlacing or locking of the rings to form a tubular stent.

[0031] To summarize, the prosthesis 50 may be a graft having the sheath52, or optionally a multi-layer graft with the sheaths 52 and 54 andother optional layer(s) or, a stent-graft, with the sheath 52 (oroptionally with the sheath 54 and other layer(s)), along with thedistensible member 56. In a preferred stent-graft embodiment, theprosthesis 50 includes the sheaths 52, 54 and the distensible member 56interposed therebetween. With the sheaths 52, 54 having wall thicknessesof 250 μm and less, the prosthesis 50 is exceptionally flexible andcapable of navigating vasculature, yet sufficiently strong to properlyfunction. The prosthesis 50 formed with the distensible member 56interposed between the sheaths 52, 54 is preferably formed with athickness in the range of 50-400 μm, although for certain applications,the prosthesis 50 may have a thickness as large as 1,000 μm. The wallthickness of the prosthesis 50, exclusive of the distensible member 56,is preferably about 120 μm or less.

[0032] It is preferred that the green tube 1 used in forming the sheaths52, 54 be extruded. With reference to FIGS. 4-5, an extruder 10 isdepicted, the extruder being more completely described in copendingapplication (Attorney Docket: 760-29). The entire disclosure ofcopending application U.S. Application Ser. No. (Attorney Docket:760-29) is incorporated herein by reference. An extruder 10 is a ramextruder formed for the extrusion of a pre-form (i.e., billet) ofpolymeric material into the green tube 1. Extruder 10 includes anextrusion mold assembly 12 (separately illustrated in FIG. 6) thatprovides a scaffolding for effective concentric support of requisiteextruder elements thereby. Assembly 12, having a forward end 12 a and arearward end 12 b, generally includes a series of platens includingstationary platens 14 and 18 and traverse platen 16. Platens 14 and 18are secured to one another in spaced alignment by a plurality of primarytruss members 19.

[0033] As shown in FIGS. 4-6, four of the truss members 19 are utilized,although other quantities may be used. It is preferred that the platens14, 18 be generally rectangular and that the truss members 19 be securedin orifices 14 a, 18 a respectively, in proximity to the corners of theplatens 14, 18. Preferably, the truss members 19 are countersunk intothe platens 14, 18, as more fully described below. With thisarrangement, strong perimetric support for the platens 14, 18 may beprovided which will maintain the platens 14,18 in a desired parallelarrangement.

[0034] The traverse platen 16 is formed with orifices 16 a and isdisposed in sliding engagement on central body portions 19 a of thetruss members 19 with the respective central body portions 19 a passingthrough the orifices 16 a. As is known to those skilled in the art,slide bearings B may be mounted in the orifices 16 a to aid in slidingmovement. It is critical to maintain parallelism between the traverseplaten 16 and the platens 14, 18. Thus, it is desired that the slidebearings B be provided with relatively small clearances relative to thetruss members 19, thus, minimizing twisting of the platen 16 duringmovement and maintaining parallelism.

[0035] Each of platens 14, 16 and 18 further includes a central aperture14 b, 16 b, and 18 b, respectively, therein to accommodate passage ofannular extruder elements therethrough. Platens 14, 16 and 18 arenotably coupled in parallel alignment so as to ensure correspondingparallel alignment therebetween for maintaining concentricity of theextruder elements throughout an extrusion cycle.

[0036] A front extruder plate 20 is also provided that similarlyincludes a plurality of orifices 20 a defined therethrough toaccommodate insertion of corresponding secondary truss members 23. Plate20 also includes a central aperture 20 b that accommodates passages ofextruder members, as further described hereinbelow. Secondary trussmembers 23 are similarly formed to the truss members 19 and are securedto the plate 20 and the platen 16 through countersunk connections tomaintain a parallel arrangement therebetween, also as described below.Truss members 23 not only provide support for plate 20, but they alsoensure parallel arrangement of the plate 20 and the platens 14, 16, 18and maintain coaxial alignment of apertures 14 b, 16 b, 18 b and 20 b.

[0037] Now referring particularly to FIGS. 4 and 5, a preferredconfiguration of concentric extruder elements may be described. Aperture14 b of plate 14 insertably accommodates a free extent of a guide rod 26therethrough. Guide rod 26 is an elongate, generally cylindrical membertraversing a length of assembly 12 in concentric relation with orifice14 b. Guide rod 26 is at least partially retained in such relation byguide rod bushing 28 that is annularly disposed in orifice 16 b oftraverse platen 16. Guide rod 26, is preferably hollow, but may beformed solid.

[0038] Guide rod 26 linearly extends in concentric relation throughorifice 16 b so as to be coaxially disposed within tubular ram 30 andmore particularly in a bore 31 defined along a longitudinal axisthereof. Ram 30, along with guide rod 26, extend in coaxial relationthrough an elongate barrel 34 having a bore 36 defined therewithin. Rambushing 32, that is positioned adjacent rear surface 18 c of plate 18,circumscribes a periphery of ram 30 so as to maintain the coaxialrelation of ram 30 relative to the barrel 34.

[0039] Ram 30 is coupled with piston 38 that is also disposed insidebore 36. An annular wiper element 39 is mounted onto the ram 30 anddisposed adjacent to piston 38 so as to provide additional concentricsupport of the guide rod 26 in relation to the ram 30 and further inrelation to the bore 36 during an extrusion operation. The wiper element39 acts as a slide bearing between the barrel 34 and the guide rod 26 inproviding support for the guide rod 26. There is tight contact betweenthe guide rod 26, the wiper element 39, and the barrel 34.

[0040] As shown in FIGS. 4 and 5 and more particularly illustrated inthe enlarged view of FIG. 7, a die mounting collar 40 having an opening40 a defined therethrough is annularly disposed about each of die 42 anda front extent of barrel 34 such that collar 40 concentrically supportsthe die 42 relative to the barrel 34. Die 42 includes a partiallytapered extrudate passage 43 therethrough that terminates in anextrudate exit opening 45 at forward end 12 b of assembly 12. Theparameters of extrudate passage 43 are partially defined by smoothsurface mandrel 48 about which a polymeric extrudate flows uponcompression of a polymeric billet by ram 30.

[0041] The dimensions of the extrudate passage 43 dictate the dimensionsof any resulting extrudate. A tip portion of the guide rod 26 definesthe mandrel 48, wherein the mandrel 48 may be unitarily formed with theguide rod 26; or, more preferably, the mandrel 48 may be formed to beremovable from the guide portion 26 with the mandrel 48 beingconcentrically supported by the remainder of the guide rod 26 by threads49 or other securement means. It may be desired to allow for easyremoval of the mandrel 48, to allow for maintenance and/or easy loadingof a billet of polymeric material (as described below). The mandrel 48may be flexible or flexibly connected to the guide rod 26 to allow forself-centering of the mandrel 48.

[0042] Referring again to FIG. 4, to further enhance secure support ofthe plate 20, tertiary truss members 33 may be mounted in proximity tothe barrel 34. The tertiary truss members 33 are preferably countersunkinto the plate 20 and the platen 14 in similar fashion as the trussmembers 19 described below.

[0043] Piston cylinders 35 are mounted to and extend between the frontplate 20 and the platen 14 with drive pistons 37 being slidably disposedwithin the piston cylinders 35. Stems 39 of the drive pistons 37 extendfrom the cylinders 35 and through the platen 18 via orifices 18 d. Thestems 39 are sealed within the orifices 18 d to prevent leakage from thecylinders 35. Ends 41 of the stems 39 are countersunk into the traverseplaten 16 in similar fashion to the securement of the truss members 19described below. Beneficially, the cylinders 35 add additional rigidityto the assembly 12, along with the countersunk connections with thetraverse platen 16.

[0044] The cylinders 35 are charged with any driving medium known tothose skilled in the art, such as hydraulic fluid or air. It ispreferred that hydraulic fluid be used and that the fluid be steadilycharged into the cylinders 35. As such the ram 30, via the traverseplaten 16, can be steadily driven towards the die 42.

[0045] In operation, a ram extrusion process is initiated in extruder 10by placing a billet or pre-form of a polymeric composition (not shown)into the bore 36 of the barrel 34. To facilitate placement of thebillet, it is preferred that the die 42 be formed to be removed from thefront plate 20. The billet can then be inserted into the barrel 34.Driving motion (i.e., via hydraulic action of drive pistons 37)transmitted to plate 16 advances plate 16 along truss members 19 towardplate 18, thereby advancing ram 30 through barrel 34. Advancement of ram30 inures translational movement to piston 38 and wiper element 39 alongguide rod 26, maintaining guide rod 26 in concentric alignment with ram30, mandrel 48, extrudate passage 43 and opening 45. Further advancementof ram 30 exerts pressure upon the polymeric billet until the billetbecomes a flowable extrudate. The extrudate flows through extrudatepassage 43 along a smooth outer periphery of mandrel 48 to define atubular profile thereby.

[0046] As indicated, various connections in the extruder 10 are achievedwith countersinking. To describe the countersinking connections,reference will be made to connections between the truss members 19 andthe platens 14 and 18. It is to be understood that the other countersunkconnections (i.e., the secondary truss members 23, the tertiary trussmembers 33, the piston stems 39) are formed dimensionally andstructurally in the same fashion. With reference to FIG. 8, each of thetruss members 19 is formed with the central body portion 19 a, having afirst diameter D₁, and end portions 22, each having a second diameter D₂smaller than the diameter D₁. As a result of this structure, annularfaces 25 are defined at the ends of the central body portion 19 a whichface outwardly. Preferably, the faces 25 are generally flat andperpendicularly disposed to a central longitudinal axis CA of the trussmember 19. Correspondingly, the orifices 14 a, 18 a are each formed witha first diameter portion 27, an annular shoulder 29, and a reduceddiameter portion 31. The first diameter portion 27 is formed with adiameter D₃ which is equal to the diameter D₁ plus a minimal clearanceδ, while the reduced diameter portion 27 is formed with a diameter D₄which is equal to the diameter D₂ plus a minimal clearance δ′. Theclearances δ, δ′ are set to allow for assembly of the elements, but itis preferred to keep such clearances to a minimum, especially theclearance δ. For example, with the diameter D₁ being equal to 2.0 inches(+0.0005/−0.0000 inches), the clearance δ is 0.001 inches(+0.0005/−0.000 inches) and thus the diameter D₃ is nominally 2.002inches; whereas, with the diameter D₂ being equal to 1.75 inches(+0.01/−0.01), the clearance δ is 0.05 inches (+0.005/−0.005), and thusthe diameter D₄ is nominally 1.85 inches. The tendency of the trussmembers 19 to shift will be a function of the clearances δ, δ′, and muchmore a function of the clearance δ′. In addition, the extent the centralbody portion 21 extends into the first diameter portion 27 not onlydictates the extent to which the truss members 19 can shift, but, also,dictates the extent to which the truss members 19 may buckle (i.e.,cause lateral deflection of the central body portion 19 a). It ispreferred that the central body portion 19 a extend a length 1 into theplaten with the length 1 being at least 33.3% of the thickness T of theplaten (e. g., 1 is at least 1 inch, where T is 3 inches). All of thecountersunk connections are formed with the same dimensionalrelationships. It is further preferred that the shoulder 29 be generallyflat and disposed perpendicularly to a central axis CA′ of therespective orifice 14 a, 18 a.

[0047] In securing the truss members 19 to the platens 14, 18, theannular faces 25 are pressed into engagement with the shoulders 29, withthe first diameter portions 27 partially accommodating the central bodyportions 19 a and the end portions 22 extending through, and extendingfrom, the reduced diameter portions 31. The bearing engagement of thetruss members 19 with the orifices 14 a, 18 a, especially at the face25/shoulder 29 interface and at the central body portion 21/firstdiameter portion 27 interface, constrains relative movement between theparts, including possible buckling of the truss members 19. Tofacilitate securing the truss members 19 to the platens 14, 18, the endportions 22 are at least partially threaded and nuts N are secured ontothe end portions 22. Any securing means known to those skilled in theart may be utilized to secure the truss members 19 to the platens 14,18.

[0048] It is further preferred that the platens 14, 16, 18 and the plate20 be each formed of sufficient thickness so as to not deflect, twist,or shift relative to the truss members 19. As an exemplary embodiment,with the platens or plate being made of AISI 1060 steel, a thickness Tof 3 inches may be provided.

[0049] With certain polymeric materials, e.g., PTFE, the tubular greenextrudate collapses upon exiting the opening 45 when extruded atthicknesses of about 250 μm or less. To overcome this problem, the tip48 b of the mandrel is formed open and in communication with an openpassage 48 a extending through the mandrel 48. A stable medium isejected from the tip 48 b into the lumen of the extrudate to pressurizethe lumen, and thus provide support. Preferably, the guide rod 26 ishollow and extends through the platen 18 allowing for a source ofcompressed air (80-100 p.s.i.g.) to be coupled thereto. The compressedair is urged the length of the guide rod 26 and through the mandrel 48.Consequently, the green extrudate is hollow and coreless upon formation.

[0050] Referring to FIG. 9, a flow chart is set forth depicting anexemplary process for fabrication of various prostheses incorporatingthin-walled extruded tubes. At initiating step 70, a polymeric paste isreceived (which may be provided in the form of a polymeric pre-form orbillet). At block 72, the billet or paste is extruded into a green,extremely thin-walled tubular profile therefrom, such as, e.g., usingthe extruder 10. At block 74, the green extruded tubing is dried(preferably heated to remove any lubricants), and thereafter expanded(block 76) to derive ePTFE having a desired node and fibrilconfiguration. The ePTFE tube is then inspected (block 78) andsubsequently cut to one or more desired lengths (block 80). This ePTFEtube may be used as an endovascular prosthesis, such as a graft. If theprosthesis is to be a stent-graft, a stent is selected and received(block 81). ePTFE tubes are used to luminally or exteriorly (or both)cover the stent (block 82), and the covered stent is subjected to asintering process such that the polymeric material fills interstices inthe stent sidewall and adheres to itself to at least partially envelopthe stent thereby; in addition, any adhesive disposed between the tubesis caused to activate (block 84). Regardless if the prosthesis is astent or a stent-graft, it is subjected to a router (block 86), cleaned(block 88), inspected (block 90), and tested for quality control (block92).

[0051] Notwithstanding the exceptional thinness of the extruded greentube disclosed herein, the thinness may even be further reduced byapplying tension to the extrudate upon formation (i.e., pull the greentube at a rate exceeding the normal exit speed of the extrudate). Thisprocess would also reduce the diameter of the green tube.

[0052] An illustrative example of the formation of first a green tube,and then a prosthesis, is provided in the example below. It isemphasized that the values provided herein merely constitute an exampleof how the present inventive process can be completed, and that thesevalues can be affected in view of material selection, desired tubethickness, piston velocity and related factors as described herein. Itis therefore understood that any or all steps may be modified insequence or duration to adapt to different applications.

EXAMPLE

[0053] ICI CD 123 resin is blended with isopar G lubricant at a ratio of15.5. to 18% isopar by weight. The resin and lubricant are blended in aV-blender for approximately 15 minutes to form a paste thereby. Apre-former that compresses the paste into pre-form (i.e. cylindricalbillets) is loaded with approximately 100 g of paste per pre-form. Thepaste is compressed into cylindrical pre-forms at hydraulic pressuresranging from 100 to 500 psi. Heat may be applied during compression inthe range from about room temperature to about 125° F. It is preferredthat the pre-forms be formed radially smaller than the bore 36 of thebarrel 34. It is desirable to obtain a clearance of 0.03125″ or less (asmeasured on diameter) between the pre-form and the barrel 34.

[0054] After removal of the pre-forms from the pre-former, the pre-formsare loaded into the extruder 10 and extruded as described above, formingan axially oriented green PTFE tube with wall thickness ranging fromabout 100 to 250 μm. The hydraulic pressures applied during extrusionrange between about 200 and about 500 psi. The pressure applied by thepiston 38 to the pre-form is in the range of about 800 to about 900p.s.i.g. The desirable reduction ratio (i.e., ratio of pre-formcross-sectional area to cross-sectional area of extruded tube) is about125 to about 350. During the extrusion, the extruder die and barrel areheated from about 77° F. to about 125° F. As the extrudate exits theextruder die, air at about 80-100 p.s.i.g. is injected into the lumen ofthe extrudate via the mandrel to maintain patency of the lumen. Upontake-up, the extrudate is lightly tensioned and cut into sections ofpredetermined length, at which point the extrudate is a “wet” greentube.

[0055] The resulting wet green tubes are placed on a tray and loadedinto a hot air drying oven at about 125° F. for 2 hours. This processremoves the isopar lubricant from the wet green tube, resulting in adried PTFE tube (hereinafter “dry” green tube). The dry green tubes arethereafter cut to a required gage length that ranges from 2.0 to 20.0 cmand mounted on expansion chucks for elongation of the tubes. A mandrelis inserted into the lumen of each of the chucked green tubes; themandrel providing a reproducible internal diameter for the desiredexpanded PTFE tube. Optionally, mandrels need not be used, wherein thedry green tubes are only loaded into expansion chucks.

[0056] The assembly of dry green tubes, expansion chucks and mandrels isloaded onto stretching trees that are used to elongate the PTFE tubesduring an expansion process. The stretching trees and assortedassemblies are placed into an oven for expansion and sintering of thedry green tubes. The dry green tube assemblies are pre-heated to about500° F. for 10 minutes, softening the PTFE for expansion. The pre-heatedassemblies are then elongated between about 2000 and about 6000% of theoriginal gage length to form expanded PTFE (ePTFE) tubes having a nodeand fibril microstructure. Velocities for the expansion range from about2.0 to about 75.0 cm/sec, and accelerations during expansion range fromabout 500 to about 5000 cm/sec².

[0057] The ePTFE tubes are then sintered at a temperature of about 660°F. for 8-14 minutes so as to elevate the PTFE to its meltingtemperature. The ePTFE tubes are thereafter cooled to room temperatureso as to preserve the dimensions and node/fibril microstructure of thetube. The cooled tubes are removed from the oven and the mandrels (ifused) removed therefrom, revealing a complete ultra-elongate thin-walledePTFE tube. Summary of ePTFE tubing: Wall thickness: 10-250 μmLongitudinal expansion: 500-6000% Internal diameters: Preferred, 1-14 mm(optionally up to 35 mm)

[0058] The present invention is thus particularly useful in producing atubular, implantable vascular prosthesis (graft or stent-graft) ofminimal outer diameter wherein the resultant tubing can be radiallyexpanded up to five times, or more, of its original diameter. Theprosthesis can be any used in connection with any of conventional andnovel artificial grafts made of various materials and adapted in variousconfigurations including straight, tapered or bifurcated and which mayor may not be reinforced with rings, spirals or other reinforcements andwhich may or may not have one or more expandable stents incorporatedinto the graft at one or both ends or along its length. The prostheticdesign of choice may be introduced into the vessel in any suitable wayincluding, but not limited to, use of a dilator/sheath.

[0059] Where the graft is expandable and in tubular or sleeve form, thediametrical size of the graft may be enlarged into contiguousrelationship with the inside surface of a vascular vessel via a ballooncatheter. The tubular graft itself may comprise a biologically inert orbiologically active anti-stenotic coating applied directly to thetreated area of the remaining vascular inner surface to define a lumenof sufficient blood flow capacity. The graft, once correctly positionedand contiguous with the interior vascular wall, is usually inherentlysecure against inadvertent migration within the vessel due to frictionand infiltration of weeping liquid accumulating on the inside arterywall. The length of the vascular graft preferably spans beyond thetreated region of the vessel.

[0060] Various changes and modifications can be made to the presentinvention. It is intended that all such changes and modifications comewithin the scope of the invention as set forth in the following claims.

What is claimed is:
 1. A green fluoropolymer tube comprising a wallhaving an inner surface and an outer surface, a thickness beinggenerally uniformly defined between said inner and outer surfaces, saidthickness being in the range of up to about 250 μm.
 2. A tube as inclaim 1, wherein said wall is seamless.
 3. A tube as in claim 1, whereinsaid tube is extruded.
 4. A tube as in claim 1, wherein said tube isgenerally cylindrical, said inner surface having a diameter of about 1mm to about 14 mm.
 5. A tube as in claim 1, wherein said tube isgenerally cylindrical, said outer surface having a diameter of about 1mm to about 14 mm.
 6. A tube as in claim 1, wherein said tube iscoreless.
 7. A tube as in claim 1, wherein said fluoropolymer is PTFE.8. A tube as in claim 1, wherein said thickness being in the range of upto about 200 m.
 9. A tube as in claim 8, wherein said thickness being inthe range of up to about 125 m.
 10. An implantable tubular prosthesiscomprising a first tube including a seamless first wall having a firstinner surface and a first outer surface, a first thickness being definedbetween said first inner and first outer surfaces, said first thicknessbeing in the range of about 250 μm or less, said first wall including anexpanded fluoropolymer.
 11. A prosthesis as in claim 10, wherein saidfirst thickness is in the range of about 200 μm. or less.
 12. Aprosthesis as in claim 11, wherein said first thickness is in the rangeof about 125 μm or less.
 13. A prosthesis as in claim 10, wherein saidfirst tube is extruded.
 14. A prosthesis as in claim 10, wherein saidfluoropolymer is PTFE.
 15. A prosthesis as in claim 10 furthercomprising a distensible member concentrically disposed relative to saidfirst tube.
 16. A prosthesis as in claim 15, wherein said distensiblemember is disposed about said first tube.
 17. A prosthesis as in claim16, further comprising a second tube disposed about said distensiblemember.
 18. A prosthesis as in claim 17, wherein said distensible memberincludes interstices, said first tube being bonded to said second tubethrough said interstices.
 19. A prosthesis as in claim 18, wherein saiddistensible member is non-bondedly disposed between said first andsecond tubes.
 20. A prosthesis as in claim 15, wherein said first tubeis disposed about said distensible member.
 21. A prosthesis as in claim15, wherein said distensible member is at least partially recessed intosaid first tube.
 22. A prosthesis as in claim 16 wherein the wallthickness of said prosthesis, exclusive of said distensible member, isabout 120 μm or less.
 23. A process for forming a prosthesis, comprisingthe step of: extruding a fluoropolymer to form a tubular member having awall with an inner surface and an outer surface, a thickness beingdefined between said inner and outer surfaces, said thickness being inthe range of about 250 μm or less.
 24. A process as in claim 23, whereinsaid fluoropolymer is PTFE.
 25. A process as claim 23, wherein saidtubular member is hollow upon extrusion.
 26. A process as in claim 23further comprising: at least partially expanding said tubular member.27. A process as in claim 23 further comprising: disposing a distensiblemember concentrically relative to said expanded tubular member.
 28. Aprocess as in claim 27, wherein said distensible member is disposed incontiguous contact with said inner surface.
 29. A process as in claim27, wherein said distensible member is disposed in contiguous contactwith said outer surface.
 30. A process as in claim 27, wherein saiddistensible member is at least partially recessed within said expandedtubular member.
 31. A process as in claim 27 further comprisingextruding a fluoropolymer to form a second tubular member having asecond wall with a second inner surface and a second outer surface, asecond thickness being defined between said second inner and secondouter surfaces, said second thickness being in the range of about 250 μmor less; at least partially expanding said second tubular member; anddisposing said second expanded tubular member concentrically relative tosaid expanded tubular member.
 32. A process as in claim 31, wherein saidfluoropolymer is PTFE.
 33. A process as in claim 31, wherein saiddistensible member is interposed between said expanded tubular memberand said second expanded tubular member.
 34. A process as in claim 33,wherein said distensible member includes interstices, said expandedtubular member being bonded to said second expanded tubular memberthrough said interstices.
 35. A process as in claim 34, wherein saiddistensible member is non-bondedly disposed between said expandedtubular member and said second expanded tubular member.
 36. A process asin claim 26, wherein said tubular member is expanded through about 500to about 6000% of elongation.
 37. A process as in claim 31, wherein saidsecond tubular member is expanded through about 500 to about 6000% ofelongation.
 38. A process as in claim 23, wherein said thickness beingin the range of about 200 μm or less.
 39. A process as in claim 38,wherein said thickness being in the range of about 125 μm or less.
 40. Aprocess as in claim 23, wherein tension is applied to said tubularmember during extruding.
 41. A process for forming a green tube, saidprocess comprising extruding a fluoropolymer to form a tubular memberhaving a wall with an inner surface and an outer surface, a thicknessbeing generally uniformly defined between said inner and outer surfaces,said thickness being in the range of about 250 μm or less.
 42. A processas in claim 41, wherein said thickness is in the range of about 200 μmor less.
 43. A process as in claim 42, wherein said thickness is in therange of about 125 μm or less.
 44. A process as in claim 41, whereinsaid tube is cylindrical, said inner surface having a diameter of about1 mm to about 14 mm.
 45. A process as in claim 41, wherein said outersurface has an outer diameter of about 1 mm to about 14 mm.
 46. Aprocess as in claim 41, wherein said tubular member is hollow uponextrusion.
 47. A process as in claim 41 further comprising injecting astable medium inside of said inner surface to provide structural supportfor said tubular member.
 48. A process as in claim 47, wherein saidstable medium is air.
 49. A process as in claim 41, wherein saidfluoropolymer is PTFE.
 50. A process as in claim 41, wherein tension isapplied to said tubular member during extruding.
 51. A method of forminga stent-graft device comprising the steps of: forming an expandablefluoropolymeric tube from an extruded green fluoropolymer tube having awall thickness of about 250 μm or less; and dispensing a distensiblemember concentrically thereabout.
 52. A method as in claim 51 furthercomprising: dispensing a second expandable fluoropolymeric tube formedfrom an extruded green fluoropolymer tube having a wall thickness ofabout 250 μm or less about said distensible member.
 53. A method as inclaim 51 further including the step of joining together said tubes. 54.A method as in claim 53 wherein said joining includes the use of heatand pressure.
 55. A method as in claim 53 wherein said joining includesthe use of adhesive.
 56. A method as in claim 51, wherein said wallthickness is about 200 μm or less.
 57. A method as in claim 56, whereinsaid wall thickness is about 125 μm or less.
 58. A method as in claim52, wherein said wall thickness of said second tube being about 200 μmor less.
 59. A method as in claim 58, wherein said wall thickness ofsaid second tube being about 125 μm or less.